1. Field of the Invention
The present invention relates to a circuit for generating a reference signal synchronized with an input burst signal. In particular, the present invention relates to a circuit for generating various reference signals of different frequencies that are adapted to different television systems, that is, different reference subcarriers for chrominance demodulation.
2. Description of the Prior Art
There are currently three well-known television systems, namely NTSC, PAL and SECAM, and especially NTSC and PAL are widely used around the world. Moreover, in addition to PAL, there are the PAL-N and the PAL-M formats. The frequencies of the reference subcarriers used for NTSC, PAL-N, PAL-M and PAL are 3.579545 MHz, 3.582056 MHz, 3.575611 MHz and 4.433619 MHz, respectively.
Conventional circuits for generating reference carriers often use automatic phase control (APC) with a phase-locked loop (PLL), as shown in FIG. 8.
In FIG. 8, a voltage-controlled oscillator (VCXO) 1 outputs an oscillation signal with the stabilized frequency f.sub.ref that is substantially determined by the resonance frequency (for example 3.58 MHz) of an external quartz oscillator 2. For simplicity, the oscillation signal with the frequency f.sub.ref is sometimes referred to as the "oscillation signal f.sub.ref " in the following. The same is true for other signals as well, where sometimes the frequency is used as the name of the signal, especially when there is no need to distinguish between the two.
A burst gate circuit 3 extracts a burst signal f.sub.SC from an input chrominance signal C.sub.in. The oscillation signal f.sub.ref and the burst signal f.sub.SC are provided as input to a phase comparator 4a of an APC circuit 4, whose output is a phase comparison signal .DELTA.f.sub.SC. After the phase comparison signal .DELTA.f.sub.SC is smoothed with a low-pass filter (LPF) 4b, it is returned as a feedback signal to the control terminal of the voltage-controlled oscillator 1.
The voltage-controlled oscillator 1 is controlled by this voltage feedback signal. As a result, the oscillation signal f.sub.ref of the voltage-controlled oscillator 1 is controlled in a manner that f.sub.ref has the same frequency and the same phase as the burst signal f.sub.SC. Thus, the continuous oscillation signal f.sub.ref, which has the same stability as the burst signal f.sub.ref can be used as the reference subcarrier signal f.sub.ref.
Since such an APC-type circuit for generating a reference carrier uses neither coils nor capacitors, it has not only mechanical stability but also stability toward temperature changes, and is suitable for integrated circuits. When a quartz is used, the free-running frequency of the oscillation signal of such a circuit for generating a reference carrier shows only small variations, and the range over which the frequency can be varied with the voltage control terminal is about .+-.500 Hz. Thus, a continuous oscillation signal with the same frequency and the same phase as the burst signal can be obtained instantly, when the chrominance signal is input.
However, because of the previously mentioned small variable frequency range, one separate quartz oscillator has to be provided for each frequency to generate several reference carriers with different frequencies.
On the other hand, if a voltage-controlled oscillator with a broad variable range (for example an RC oscillator) is used for generating several reference carriers with different frequencies, the time from free running until an oscillation signal with the same frequency and the same phase as the burst signal can be obtained--that is, until the APC locks--is long. Moreover, so-called "side-locking" can occur, where a frequency near the targeted oscillation frequency, but one line frequency away, is locked.
In order to avoid side-locking, variations in the oscillation frequency have to be suppressed and the variable frequency range has to be narrowed, but then it becomes difficult to generate several reference carriers with different frequencies. Moreover, it is difficult to include RC oscillators on integrated circuits, because in ICs, the variations in the resistance R and the capacitance C are large.
To generate a plurality of reference carriers with different frequencies adapted to a plurality of television systems as described above, Publication of Examined Japanese Patent Application No. Sho 63-28521 discloses the use of several different external quartz oscillators, one for each frequency, which can be switched accordingly.
Furthermore, when the passband of the lowpass filter 4b in the APC circuit using the PLL is narrow, the time until synchronization is long, resulting in deterioration of stability. Publication of Unexamined Japanese Patent Application No. Sho 63-82084 discloses a configuration for increasing the precision of the oscillation frequency while increasing the stability by widening the passband of the lowpass filter. This conventional example uses one quartz oscillator each for two voltage-controlled oscillator circuits, and increases the stability of the oscillation frequency by using a dual loop PLL.
However, in these conventional technologies the circuits become rather complicated, resulting in the problem of increasing size and costs. Thus, it has been desired strongly to solve those problem, particularly in a field of appliances where the demand for lower costs and miniaturization is high.